fix(feed): format RSS build dates

lib/pleroma/web/feed/feed_view.ex

@@ -38,6 +38,20 @@ defmodule Pleroma.Web.Feed.FeedView do
     end
   end
 
+  def most_recent_update([], :rss), do: nil
+
+  def most_recent_update(activities, :rss) do
+    activities
+    |> Enum.map(fn activity ->
+      case Object.normalize(activity, fetch: false) do
+        %Object{updated_at: updated_at} -> updated_at
+        _ -> activity.updated_at
+      end
+    end)
+    |> Enum.max_by(&NaiveDateTime.to_erl/1)
+    |> to_rfc2822()
+  end
+
   def most_recent_update(activities, user, :atom) do
     (List.first(activities) || user).updated_at
     |> to_rfc3339()

lib/pleroma/web/templates/feed/feed/tag.rss.eex

@@ -12,7 +12,9 @@
     <webfeeds:logo><%= feed_logo() %></webfeeds:logo>
     <webfeeds:icon><%= feed_logo() %></webfeeds:icon>
     <webfeeds:accentColor>2b90d9</webfeeds:accentColor>
-    <lastBuildDate><%= most_recent_update(@activities) || to_rfc2822(NaiveDateTime.utc_now()) %></lastBuildDate>
+    <%= with last_build_date when not is_nil(last_build_date) <- most_recent_update(@activities, :rss) do %>
+      <lastBuildDate><%= last_build_date %></lastBuildDate>
+    <% end %>
     <generator><%= Pleroma.Application.named_version() %></generator>
     <%= for activity <- @activities do %>
     <%= render Phoenix.Controller.view_module(@conn), "_tag_activity.xml", Map.merge(assigns, prepare_activity(activity)) %>